The present invention relates to frequency synthesizing devices and it relates, more particularly, to such devices wherein periodic electrical oscillations are arbitrarily determined at a frequency that is changed and reset to a desired value while maintaining phase coherence.
In radio and radar technology, transmitters are employed utilizing transmitting frequencies that are changed rapidly over large frequency spans. A rapid change of frequencies makes it possible to dispense with, for example, powerful, narrow-band interfering transmitters. In order to obtain a useful signal with a minimum of interference, in this case the signals arriving at the receiver should be demodulated in a phase-coherent manner. A prerequisite for phase-coherent demodulation is that the phase position of the transmission signals and the reference signals fed to the demodulator return to a predetermined value after each change of frequency.
U.S. Pat. No. 3,696,306 discloses a phase-coherent frequency synthesizing device for generating an electrical oscillation whose phase position always takes on a predetermined value after rapid, drastic changes in frequency. The disadvantages of this circuit, which is constructed in analog technology, are the large amount of circuitry required and a relatively long switching time for a change of frequency. In addition, a small line interval for the resulting frequency allocation scheme can be obtained only by means of extensive additional circuitry.
In the journal "RF Technology" (1987) 5, the article entitled "Direct Numerical Synthesis of Sine Waves" by Fred Williams describes the construction of a direct digital frequency synthesizing device which permits electrical oscillations to be generated in the time range. This digital frequency synthesizing device has a simple circuit arrangement, which can be obtained with a few integrated building blocks. However, the frequency switching in this device is phase-continuous, that is, the final value of the phase .phi..sub.A of the previously generated frequency f.sub.A is equal to the starting value of the phase .phi..sub.N of the new frequency f.sub.N.